> Along these same lines, a tool which would generate a symbol usage DAG> for an entire (very large) program could be extremely valuable.

Such a thing exists, but as far as I know it's not available. See
Yih-Farn Chen, "The C Program Database and Its Applications", USENIX
Summer 1989. There is a C++ equivalent: Judith Grass and Yih-Farn Chen,
"The C++ Information Abstractor", 1990 USENIX C++. These are both fine
papers, some of my favorites in the field of C/C++ programming
environments.

I think CIA (the C tool) has been used on multi-million-line programs.
Too bad AT&T never made it a product.